With self-propelled work machines such as dump trucks, trucks, bulldozers or self-propelled cutters such as surface miners, snow blowers or asphalt cutters, electric drives having at least one electric motor have been used in recent times to utilize the typical advantages of such electric drives with respect to hydrostatic drives such as their better efficiency and an easier maintenance. Considerably lower operating costs can also be achieved in the partly substantial powers due to the substantially better efficiency. The electric drive can in this respect in particular be utilized as a traction drive by means of which at least one wheel or one chain drive of the undercarriage is driven, but also for driving a main work unit such as the milling cutter of a surface miner.
In this respect a generator can be provided for the power supply of the electric drive and can be driven by an internal combustion engine, for example in the form of a diesel engine, a gasoline engine or a gas engine. The power electronics between the named generator and a respective electric motor with which, for example, a wheel of the chassis or a pinion of a chain drive can be driven, in this respect as a rule comprises two transformers of which one serves as a generator transformer and is associated with the generator and the other serves as a motor transformer and is associated with the electric motor, wherein the two transformers can be connected by a common intermediate circuit, in particular a DC voltage intermediate circuit. The electric motor is supplied from the generator with electric power via the named power electronics, with, optionally with a bidirectional configuration of the power electronics, a feedback of electrical motor braking power to the generator being able to take place which is generated by the electric motor in coasting operation. Such a transformer can be formed, for example, as a frequency inverter or as a DC-DC controller.
Since the use of such diesel-electric drive systems have previously not been widespread in construction machinery or mining machinery such as trucks, dump trucks, crawler-type vehicles and the like, the service personnel present at the machine operators usually only have basic electrical knowledge such that serving and repair work is as a rule only restricted to the replacement of individual components. In this respect, due to the limited technical electrical knowledge of the service personnel, safety problems occasionally occur which may lead to serious danger situations due to the high powers and high voltages.
Considerable residual charges and residual voltages occasionally remain in the electrical drive components of such work machines even if the work machine had already been turned off or switched off for some time. A discharge typically takes place via the braking resistors, which, however, as a rule does not result in a complete discharge and does not exclude the named residual voltages. In this respect, as a rule, the absence of voltage must be checked by electrically trained technical personnel using corresponding measurement devices before work can be carried out at the electrical drive components. In this respect, a grounding of the components is likewise carried out by electrically trained technical personnel using external grounding devices, which, however, exceeds the routine and experience of the normal service personnel of machine operators.
Construction machinery using such diesel electrical drive systems are known, for example, from the documents U.S. Pat. No. 7,950,481 B1 and U.S. Pat. No. 8,395,335 B2.
It is the underlying object of the present invention to provide an improved work machine of the initially named kind which avoids disadvantages of the prior art and further develops the latter in an advantageous manner. A safe replacement of electrical drive components should in particular also be made possible for personnel not trained extensively in electrical work.
In accordance with the invention, the named object is achieved by a work machine having an electric drive comprising power electronics which have at least one transformer and/or an intermediate circuit, and having power connections covered by a cover for connecting power cables, wherein a manually actuable grounding device is provided for grounding of the transformer and/or for short-circuiting of the at least one intermediate circuit, characterized in that wherein the grounding device is coupled to a cover latching for latching the cover such that the cover is unlatched by actuating the grounding device; and by a transformer for a work machine having an electric drive, the transformer having power connections covered by a cover for connecting power cables, wherein a manually actuable grounding device is provided for grounding of the transformer and/or for a short-circuiting of at least one intermediate circuit, wherein the grounding device is coupled to a cover latching for latching the cover such that the cover is unlatched by actuating the grounding device.
It is therefore proposed to configure the transformer such that, on an unlatching of the cover by which the connection regions of the components endangered by residual voltage, the grounding device has to be automatically actuated such that a grounding or absence of voltage is present when the cover can be removed. In accordance with the invention, the manually actuable grounding device is coupled to a cover latch of the cover such that the cover can be unlatched by actuating the named grounding device. The unlatching of the connection regions endangered by residual voltage takes place by the grounding device so that a grounding is ensured before use is made of the named connection regions, for example to release power cables. The replacement of the corresponding component can also be entrusted to technical personnel without particular electrical training thanks to such a grounding with compulsory control.
The induced grounding is in this respect advantageously of an all-pole type. An all-pole type grounding in this respect means the all-pole connection of the active conductors with the protective ground or with the conductive chassis or housing of the system. The all-pole grounding of the intermediate circuit (DC) can be sufficient with converters. The grounding of the alternating current connections (AC) can be provided by free-running diodes of the power semiconductor (IGBT) or diodes of rectifiers of the power electronics. The introduction of electrical energy can hereby be prevented during the replacement of one or more transformers of the system.
In a further development of the invention, the coupling between the grounding device and the cover latch is configured in this respect such that the cover can actually only be opened when the grounding device is in its grounded position. If the grounding device is not in its grounded position or if it was not yet actuated, the cover latching is held in its latching position so that the cover cannot be released. In an alternative further development, it would admittedly likewise be possible to provide an expert unlatching by means of which trained technical personnel can also open the cover, for example, with special tools even with an unactuated grounding device. To avoid an incorrect operation which endangers safety, however, the previously named compulsory coupling between the grounding device and the cover latching can be advantageous which holds the cover latching in the latched position as long as the grounding device has not been actuated and is in an ungrounded position.
The coupling between the grounding device and the cover or its cover latching can be configured mechanically in a further development of the invention. For example, an actuation lever of the grounding device can be connected to a latching part of the cover latch or can itself form the named latching part such that the latching part is always co-moved when the actuation lever of the grounding device is moved. Alternatively or additionally to such a mechanical coupling, the cover latching can also be electrically and/or electromagnetically coupled to the grounding device, for example such that a latching part is electromagnetically brought into the latched position and is held there, for example for so long as a sufficiently high residual voltage is still present in the transformer intermediate circuit, with the latching part then being able to move into the released position, for example by means of a spring device, when the residual voltage has dropped. Such an electrical or electromagnetic control of the cover latching can also work in dependence on the grounding device, for example such that the voltage actuating the electromagnet and thus latching the latching part is interrupted or reduced by the grounding device when the grounding device is brought into its grounded position.
To ensure that the grounding device remains in the grounded position after releasing the cover or during servicing, dismantling or removing of the component, the grounding device can advantageously be latched in the grounded position. For this purpose, a suitable latch element or a lock can be provided which can advantageously be configured as self-latching, for example in the form of a biased snap-in latch which latches when the grounding device has been manually actuated.
In a further development of the invention, locking and/or unlocking means can be associated with the grounding device by means of which the grounding device can only be actuable, i.e. can only be brought into the grounded position, when the residual voltage in the system does not exceed a predefined voltage limit, for example is below a safety extra-low voltage limit. If the residual voltage in the system still exceeds the named voltage limit, the grounding device remains unactuable so that the system cannot be grounded at voltages which are too high.
In a further development of the invention, the transformer can comprise a residual voltage gauge for measuring a residual voltage, wherein the grounding device has an actuation lock for locking the grounding device in the ungrounded position in dependence on the measured residual voltage. The locking of the grounding device can in this respect be effected in the sense of a blocking such that a grounding element cannot be moved into its grounded position. Alternatively or additionally, the lock can also interrupt an actuation train between the manual actuation lever and the grounding part such that an actuation of the actuation element of the grounding device is unsuccessful.
The named residual voltage gauge can advantageously be integrated into the transformer, for example can be implemented by an integrated voltage measurement circuit.
The named residual voltage gauge can advantageously be connected to a residual voltage display for displaying the still remaining residual voltage to indicate to a service mechanic that the residual voltage has still not been sufficiently reduced and/or has already been sufficiently reduced. Such a residual voltage display can, for example, comprise an optical display apparatus at the housing exterior, for example in the form of a color LED.
In order not to have to connect any external discharge device to the transformer, in a further development of the invention, an automatic discharge device can be integrated into the transformer which automatically discharges the transformer or a transformer intermediate circuit, for example every time after switching off the work machine and/or after an absence of a control signal to the transformer, for example when the control cable is removed during servicing. A compulsory controlled, two-stage servicing process can be achieved by such a discharge device which automatically discharges the power electronics or their transformer every time after a turning off of the work machine and/or after inputting a maintenance or service command and/or after removing the control cable until the residual voltage falls below the aforesaid predefined voltage limit. The work machine first has to be turned off and/or the command has to be given that the work machine is in the service mode and/or the control cable has to be removed from the transformer. This causes the named discharge device to automatically discharge the transformer or the intermediate circuit. Only when the discharge process was so successful that the residual voltage gauge determines a sufficiently small residual voltage is the grounding device unlocked so that a service engineer can close the grounding device or can bring it into the grounded position. This, i.e. the actuation of the grounding device, in turn releases the cover latching so that the cover can be released or removed and thus access can be achieved to the connection regions endangered by residual voltage.
The electrical drive system or the electric drive of the work machine can in this respect have a plurality of transformers which can advantageously all be protected in the named manner by a grounding device and/or by a discharge device. For example, the electrical drive system can comprise at least one motor transformer associated with an electric motor and one generator transformer associated with a generator which are connected to one another by a common intermediate circuit to supply the electric motor with electrical energy from the generator in the drive mode and/or to feed motor braking power generated by the electric motor in coasting operation to the generator and optionally to apply it to an internal combustion engine which drives the named generator to generate the electrical current in the drive mode.
A redundancy can be achieved in a common system by the use of two transformers which can each be provided in the named manner with a grounding device and with a discharge device. The other electrical transformer advantageously serves as a system short-circuiting device after removal of an electrical transformer. The grounding of the remaining transformer ensures that the system remains grounded overall even if a transformer has been removed.
Due to the removal of at least one transformer from the system, the release for the start of the internal combustion engine can be latched such that a restart of the internal combustion engine is suppressed and is only present again after reinstallation of all the transformers. The introduction of electrical energy can hereby be prevented during the replacement of one or more transformers of the system.
The internal combustion engine start latch can be formed, for example, in the form of an electrical unlocking chain analog to an emergency stop chain. Before the removal of at least one transformer, inter alia all the electrical connections thereto should be separated. The unlocking chain is hereby separated and the electrical circuit to the starter device of the internal combustion engine is, for example, interrupted. Only after the reinstallation and connection of all the transformers of the system is the unlocking for starting the internal combustion engine present again.
The invention will be explained in more detail in the following with reference to a preferred embodiment and to associated FIGURES.